1. Field of the Invention
This invention relates to an apparatus and method for use in infrared imaging. More specifically, a pattern of infrared radiation is detected by and stored in substantially uniformly distributed localized states in a semiconductor.
2. Description of the Prior Art
Various types of radiation detectors and detection methods employing charge storage are known in the art.
U.S. Pat. No. 3,341,825, for example, discloses a quantum mechanical information storage system in which storage and retrieval of information is accomplished by scanning a storage medium with a beam of radiant energy. A bit of information is written or stored on a particular location on the material by a beam of light of a particular frequency or wavelength and is retrieved from the material with a beam of light of a different wave length. The memory material consists of a semiconductor material into which certain impurities are introduced to act as energy level traps for electrons. A light beam of a particular frequency impinging on the memory material at a particular position causes the energy level of electrons at that position to be raised from the valence level to a higher level called the storage level. The electrons remain at the storage level until they are raised to the conduction level by the action of a light beam of a frequency different from the first.
Additionally, U.S. Pat. No. 4,167,791 discloses a semiconductor memory device exhibiting nonvolatile information storage. The device comprises a storage element, maintained in a prescribed range of temperatures, which exhibits an effect of charge storage and release of the stored charge upon application of a suitable bias voltage. The device is, for example, exemplarily made of silicon in which the rectifying region consists of a region of highly doped p-type material and a region of highly doped n-type material and, in between the highly doped regions, a region of very lightly doped material. Information is stored by exposure to light or by applying a bias voltage to put the device into one of a multiplicity of long lived states. In the case of exposure to light the state of the device is indicative of the integrated photon flux. Shining light on the device at the initial bias, before applying a voltage sweep, results in a larger pulse being caused by said voltage sweep, although the pulse never exceeds a certain saturation size. Under appropriate conditions the size of the pulse is a monotonically increasing function of the total amount of light incident on the device.
The effect of the initial exposure to radiation in each of the prior art devices hereinbefore described is to cause an increase in the amount of charge stored in such devices. The effect of exposure to a pattern of infrared radiation with the present invention, on the other hand, is to cause a depletion of stored charge.
A major problem with detection systems which measure electrical signal output during exposures to radiation is that it is extremely difficult to measure low intensity infrared radiation with such devices because the electrical signal output is exceedingly small. To overcome this problem, external integration of the output signal may be utilized to accumulate a larger, more detectable signal over a period of time. With presently known integrating detectors, however, deleterious noise and other problems are introduced by the external transfer of information.
A problem with detection systems based on an increase in stored charge such as is described in the hereinbefore patents is that such systems require relatively large photon energy levels to excite carriers which can be trapped in localized states associated with typical impurity levels to cause an increase in the stored charge. As a result, for a particular localized state in a particular semiconductor, the wavelength range within which the amount of stored charge can be altered by radiation exposure is limited to relatively short wavelengths.
In addition to the radiation detection devices and methods described above, various types of infrared and other electromagnetic imaging devices are known. U.S. Pat. Nos. 3,182,198; 3,398,316; 3,652,270 and 4,265,991, for example, disclose some of these devices. None of these patents, however, teach or suggest an infrared imaging device in which an image or pattern of radiation is stored in the semiconductor by depletion of charge stored in localized states in a semiconductor caused by field-assisted photoemission. In U.S. Pat. No. 3,652,270 a dispersed mixture of material is utilized as a recording device. That system, however is not capable of providing or approaching the same degree of homogeneity as is provided by the present invention.
There remains, therefore, a need for an imaging device and method which is capable of detecting low intensities of infrared radiation while avoiding the problems associated with external integration of the output current. There remains a further need for an imaging device and method which provides in situ integration of radiation with lower photon energy to allow for the detection of much longer wavelengths of radiation with any particular semiconductor. There also remains a further need for an imaging device and method which can detect and store a pattern of infrared radiation in a single uniform semiconductor based on the depletion of charge stored in localized states of a semiconductor caused by field-assisted photoemission of charge carriers from the localized states.